Add newer and anyNewer options to file.expand

The newer option takes a date and will limit the expanded files to those with greater mtime dates.  If the anyNewer option is true, all matching files will be returned if any one is newer.
Tests for anyNewer option
Keep track of last run time and modify newer option

This adds support for a "newer" option for files.  If true, only files modified since the last run will be matched.  If false (the default), files are matched in the normal way.  If the newer option is "any", all files will be matched if any one of them has been modified since the last run.
Safe path joining for 0.10